1. Field of the Invention
This invention relates generally to implantable medical devices and relates more particularly to arrangements for communicating data between a first medical device and an associated second medical device.
2. Background Information
Modern implantable medical devices such as cardiac stimulators, including, inter alia, pacemakers and defibrillators, are controlled by programmable microprocessors that can be reprogrammed before and after implantation of the device in a patient. This permits the physician to change the parameters and modes of operation of the implantable device to suit the needs of the patient. Such devices also are capable of reporting various aspects of their own status as well as sensing various physiological parameters of the patient that are useful to the physician. Consequently, it is necessary to provide means for communicating information, such as instructions and data, to and from the implantable medical device, both before and after implantation. This is often accomplished with an associated non-implantable medical device, sometimes referred to as a programmer, that provides an appropriate interface to the physician. The physician interface includes input and output devices, such as keypads, light pens, display screens, and printers. The non-implantable device, or programmer, also includes an interface for communicating with the implantable medical device, such interface often taking the form of a wand that can be placed near the implantable medical device to provide communication over a short distance.
A commonly employed arrangement for providing communication over a short distance, both before and after implantation, involves coupling the implantable and non-implantable medical devices through a magnetic field generated by one or the other of the medical devices. The transmission of information between the devices is accomplished by modulating the magnetic field in such a way as to encode the information to be transmitted. Amplitude modulation, frequency modulation, pulse width modulation and pulse position modulation are commonly used means for modulating the magnetic field. In a typical arrangement, a coil of wire in the implantable medical device carries a transient current that is driven by a communications circuit within the implantable device and modulated according to the information to be encoded and transmitted from the implantable device to the non-implantable device. A varying magnetic field, modulated according to the information encoded therein, is generated by the transmitting coil for interception by a receiving coil of wire in the non-implantable medical device. The magnetic field generates a transient current in the receiving coil. The receiving coil is connected to appropriate amplifying, demodulating and decoding circuitry for processing the received transient current. Likewise, the coil of wire in the implantable device can function as a receiving coil for intercepting a magnetic field generated by the non-implantable device, in connection with appropriate amplifying, demodulating and decoding circuitry in the implantable device. As an alternative, separate transmitting and receiving coils can be used in either or both of the medical devices.
In the communications arrangement discussed above, the coupling between the respective coils of the implantable and non-implantable devices is primarily magnetic. A disadvantage of such arrangements is their sensitivity, over a relatively great distance, to noise from stray magnetic fields generated by medical equipment that is often present in health care facilities. The rate at which information can be communicated between the magnetically coupled coils of the implantable and non-implantable medical devices is limited by the presence of such magnetic noise. Also, the relatively high inductance of the coils limits the maximum frequency at which the magnetic field can be modulated, thereby also limiting the maximum rate at which information can be communicated. Another disadvantage is that a significant portion of the energy delivered to the transmission coil is dissipated as heat due to ohmic resistance of the coil wire. In the implantable device, the energy used for communication is usually drawn from a battery hermetically sealed within the implantable device, which battery also supplies the energy for the therapeutic stimulating pulses that are generated by the device. Consequently, it is important to minimize the energy consumed for communication purposes in order to preserve the therapeutic life span of the implantable device.
It would be desirable to provide an arrangement for communicating information over a short distance between a first medical device and a second medical device having improved immunity to ambient magnetic noise.
It also would be desirable to provide an arrangement for communicating information over a short distance between a first medical device and a second medical device that permits an increased rate of transmission of data.
Furthermore, it would be desirable to provide an arrangement for communicating information over a short distance between a first medical device and a second medical device that has improved energy efficiency.
These and other desirable advantages are provided by the present invention.